If you're experiencing frame rates much below 60fps, then mouselook will feel more sluggish (because the game itself is sluggish). If you open the console with backquote/tilde and enter "tdbg" (without quotes), what framerate are you getting during play? If this is capped at 60, do you experience more precise mouse movement with vertical sync disabled?

In any case, it's very simple to allow for higher sensitivity settings. I've set the cap on this to 8.0 now. This will be available in next round of builds.